Because the commonest element of the ship's structure is the orthogonally stiffened plate, the investigation on such structure is indispensable for designing, for instance, deck, bottom and bulkhead of ship. In this first report, the theoretical solutions of orthotropic plates with various end conditions under various lateral loads are given employing the general solution of the basic equation, for the sake of analysing the orthogonally stiffened plate. Some considerations are also performed in case added stiffener is installed on stiffened plate, as the center girder on the bottom in actual ship.